A stick-on patch the size of a credit card uses a tiny electric current to deliver pain medication through the skin, according to U.S researchers.

In a trial published in today's issue of the Journal of the American Medical Association, the patch delivered a painkiller to people who had just had major surgery about as well as a conventional intravenous device.

If future trials are successful, the transdermal technology could be an alternative to the needles, tubes and a pump presently used to deliver painkillers such as morphine after surgery.

The patches could also allow patients to move about more freely, the researchers said.

Like a conventional intravenous device, the patch allows patients to control how much medication they receive. This patient-controlled analgesia delivers small amounts of opioids like morphine.

The patch works by transferring a hydrogel reservoir of the painkiller into the skin. The drug then diffuses into the local circulation and is transported to the central nervous system.

The transfer through the skin occurs by a process called iontophoresis, in which a charged substance is propelled by a low electrical current.

Patients administer their own medication using the patch by pressing a dosing button on the surface of the device twice within three seconds.

The system has a 'lock out' feature so patients cannot administer more painkillers than doctors prescribe.